The likely culprit is a mismatch between JSON syntax and the front-matter syntax used earlier in the tutorial.
The two syntaxes look similar but aren't interchangeable. Front matter (the `---` block at the top of an `.md` or `.html` file) is YAML, so unquoted keys like `layout: layout.html` are fine.
A `posts.json` file is *actual JSON*, which requires double quotes around every key and string value, and no trailing commas. If it was typed the same way as front matter, `{ layout: layout.html }` then that's invalid JSON and Eleventy either throws a parse error or silently fails to apply it, which matches what you're describing (title not filling in, index not building).
Here's a working reference setup:
**`posts/posts.json`**
```json
{
"layout": "layout.html",
"tags": "post"
}
```
Two things here, `"layout"` applies `layout.html` to everything in the folder, and `"tags": "post"` is what makes `collections.post` populate on the homepage. If your version only has `layout` and not `tags`, the layout/title problem and the missing index are actually two separate symptoms of one missing line.
**`posts/post-1.md`**
```markdown
---
title: My First Post
---
This is the content of my first post.
```
The `title` still has to be set per-post in that file's own front matter, `posts.json` assigns the layout to every file in the folder, but it doesn't invent a title for each one. If a post's front matter is missing `title:`, `{{ title }}` in the layout will just render blank.
**`_includes/layout.html`**
```html
{{ title }}
{{ title }}
{{ content }}
```
**`index.html`** (homepage, listing posts)
```html
---
layout: layout.html
title: My Blog
---
```
Also:
- Wipe `_site` and restart the server. You already saw stale folders stick around after renaming `test-post.md` to `post-1.md` 11ty doesn't always clean up old output on renames. `Ctrl+C` the terminal running the local dev server, delete the `_site` folder, then run `npx @11ty/eleventy --serve` to restart.
- Watch the terminal for a JSON parse error. If `posts.json` is malformed, Eleventy will print something like `Error: posts.json: Unexpected token` on save.